Alternative splice variants of the mitochondrial fission protein DNM1L/Drp1 regulate mitochondrial dynamics and tumor progression in ovarian cancer.

Aberrant mitochondrial fission/fusion dynamics have been reported in cancer cells. While post translational modifications are known regulators of the mitochondrial fission/fusion machinery, we show that alternative splice variants of the fission protein Drp1 (DNM1L) have specific and unique roles in cancer, adding to the complexity of mitochondrial fission/fusion regulation in tumor cells. Ovarian cancer specimens express an alternative splice transcript variant of Drp1 lacking exon 16 of the variable domain, and high expression of this splice variant relative to other transcripts is associated with poor patient outcome. Unlike the full-length variant, expression of Drp1 lacking exon 16 leads to decreased association of Drp1 to mitochondrial fission sites, more fused mitochondrial networks, enhanced respiration, and TCA cycle metabolites, and is associated with a more metastatic phenotype in vitro and in vivo. These pro-tumorigenic effects can also be inhibited by specific siRNA-mediated inhibition of the endogenously expressed transcript lacking exon 16. Moreover, lack of exon 16 abrogates mitochondrial fission in response to pro-apoptotic stimuli and leads to decreased sensitivity to chemotherapeutics. These data emphasize the significance of the pathophysiological consequences of Drp1 alternative splicing and divergent functions of Drp1 splice variants, and strongly warrant consideration of Drp1 splicing in future studies.

HuR-dependent SOD2 protein synthesis is an early adaptation to anchorage-independence.

During metastasis cancer cells must adapt to survive loss of anchorage and evade anoikis. An important pro-survival adaptation is the ability of metastatic tumor cells to increase their antioxidant capacity and restore cellular redox balance. Although much is known about the transcriptional regulation of antioxidant enzymes in response to stress, how cells acutely adapt to alter antioxidant enzyme levels is less well understood. Using ovarian cancer cells as a model, we demonstrate that an increase in mitochondrial superoxide dismutase SOD2 protein expression is a very early event initiated in response to detachment, an important step during metastasis that has been associated with increased oxidative stress. SOD2 protein synthesis is rapidly induced within 0.5-2 h of matrix detachment, and polyribosome profiling demonstrates an increase in the number of ribosomes bound to SOD2 mRNA, indicating an increase in SOD2 mRNA translation in response to anchorage-independence. Mechanistically, we find that anchorage-independence induces cytosolic accumulation of the RNA binding protein HuR/ELAVL1 and promotes HuR binding to SOD2 mRNA. Using HuR siRNA-mediated knockdown, we show that the presence of HuR is necessary for the increase in SOD2 mRNA association with the heavy polyribosome fraction and consequent nascent SOD2 protein synthesis in anchorage-independence. Cellular detachment also activates the stress-response mitogen-activated kinase p38, which is necessary for HuR-SOD2 mRNA interactions and induction of SOD2 protein output. These findings illustrate a novel translational regulatory mechanism of SOD2 by which ovarian cancer cells rapidly increase their mitochondrial antioxidant capacity as an acute stress response to anchorage-independence.

Emerging perspectives on growth factor metabolic relationships in the ovarian cancer ascites environment.

The ascites ecosystem in ovarian cancer is inhabited by complex cell types and is bathed in an environment rich in cytokines, chemokines, and growth factors that directly and indirectly impact metabolism of cancer cells and tumor associated cells. This milieu of malignant ascites, provides a 'rich' environment for the disease to thrive, contributing to every aspect of advanced ovarian cancer, a devastating gynecological cancer with a significant gap in targeted therapeutics. In this perspective we focus our discussions on the 'acellular' constituents of this liquid malignant tumor microenvironment, and how they influence metabolic pathways. Growth factors, chemokines and cytokines are known modulators of metabolism and have been shown to impact nutrient uptake and metabolic flexibility of tumors, yet few studies have explored how their enrichment in malignant ascites of ovarian cancer patients contributes to the metabolic requirements of ascites-resident cells. We focus here on TGF-ßs, VEGF and ILs, which are frequently elevated in ovarian cancer ascites and have all been described to have direct or indirect effects on metabolism, often through gene regulation of metabolic enzymes. We summarize what is known, describe gaps in knowledge, and provide examples from other tumor types to infer potential unexplored roles and mechanisms for ovarian cancer. The distribution and variation in acellular ascites components between patients poses both a challenge and opportunity to further understand how the ascites may contribute to disease heterogeneity. The review also highlights opportunities for studies on ascites-derived factors in regulating the ascites metabolic environment that could act as a unique signature in aiding clinical decisions in the future.